1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium comprising a recording layer containing a dye, a reflective layer and a protective layer, and capable of recording a large volume of information and playable by commercially available compact disc players.
2. Description of the Related Art
As an information-oriented society has recently developed rapidly, optical recording mediums have been widely utilized and researched which can make a remarkably high density recording as compared with magnetic recording mediums.
As optical recording mediums, there are known a read-only-medium in which information has been recorded in advance and only reproduction is possible, a writable medium in which information can be recorded by users and reproduced, and a rewritable medium in which information can be recorded therein and the information thus recorded can be reproduced or erased.
Among them, the writable medium is generally inexpensive and excellent in durability as compared with the rewritable medium. Therefore, the writable medium has been widely used as a medium for storing a large volume of data.
The optically writable medium may have a metal layer of Te, Bi or the like or a dye layer of cyanine dyes, phthalocyanine dyes or the like as a recording layer on a substrate.
Information can be recorded in the recording layer by irradiating a laser beam to cause a physical or chemical change (pits formation) in the layer.
The pits are read out with a laser beam which is sufficiently weaker than that used for recording and thereby the information can be reproduced.
Among the optically writable mediums, there have recently been developed and spread some mediums capable of reading out by commercially available compact disc (CD) players.
The writable mediums which can be reproduced by CD players are proposed in Optical Data Storage 1989 Technical Digest Series Vol. 1, 45 (1989), EP-353393 and the like. The mediums are of a single plate type comprising a recording layer composed of an organic dye, a metal reflective layer and a protective layer.. However, a cyanine dye is used in the recording layer so that there are problems as to poor light resistance and poor heat and humidity resistance.
Further, EP - 0373643 discloses a writable medium comprising a recording layer composed of phthalocyanine type dye which is excellent in light resistance and humidity and heat resistance and can be reproduced by a CD player.
When an optical disc is manufactured which has a pit and a track in a rugged form, the molding shrinkage varies in the radial direction depending on a material used for forming a substrate and molding conditions. For the purpose of solving this problem, Japanese Patent Application Laid-open No. 131654/1985 proposes to brooden the width of pits in the inner area of a stamper taking the molding shrinkage into consideration.
Further, optical discs of CAV (constant angular velocity) mode such as video disc and the like have a drawback that reflection conditions upon recording differ. In order to eliminate the drawback, Japanese Patent Application Laid-open No. 282758/1989 proposes that the depth of pits at the inner area of a substrate is made deeper than that at the outer peripheral area.
In the case of the above-mentioned medium having a recording layer composed of a dye, the recording layer is usually formed by coating an organic dye solution, in particular, a spin-coating method is generally used since the film forming is easy and the method is economical.
However, when a recording layer is formed by a spin-coating method, the present inventors have found that it is very difficult to coat uniformly a recording layer on a substrate having a groove. In particular, the thickness of the recording layer on a groove and that on a land (a portion between adjacent grooves) vary from the center portion of the substrate toward the outer periphery of the substrate, and the characteristics such as reflectivity, push-pull signal, radial contrast and the like are disadvantageously non-uniform in the direction from the center portion of the substrate to the outer periphery thereof, since such characteristics depend on the thickness of the recording layer on a groove and that on a land.
The present inventors have intensively made researches for solving the above-mentioned problems. As a result, they have found that the above-mentioned characteristics largely depend on the surface shape of the recording layer and the shape of the groove and completed the present invention.